Main earth bonding

[EFLImpudence]

Therefore to apply earthing to parts which cannot become live because they are not part of an electrical appliance or accessory is not beneficial.
Some seem to think that live conductors can suddenly jump out and attack random pipes - we do not have to cater for this.

Now we take a hypothetical installation where the sparky has been round and done his bit. And the sparky determined that the water pipes didn't need any earth or bonding connection. What's more, none of his cables are on/close to any pipes.
Then the plumber comes round and puts the plumbing in, and like many plumbers, doesn't seem to care about cables. So somewhere there's a cable sat on a hot pipe. Over time, with the heat and movement, the insulation slowly wears through until fate decided that the live conductor which happened to be on (the wrong side) contacts the pipe.

Because someone didn't cater for it, we now have live pipework with only the boiler wiring to "earth" it - hence an inadequate fault current to make the breaker trip in a timely manner.
But it's alright if anyone dies in the ensuing fire, because it wasn't the sparkies responsibility to account for that - or that's how I read your statement.

It is a compromise. Nothing is perfect.
If you want to earth all your isolated metal parts, I suppose that is up to you.
You then happen to hold a faulty appliance flex while touching one of these (wrongly) earthed parts and get a shock greater than you should.
What then?

Of course, these days the circuit will almost certainly have RCD protection. That's not the case in older properties that haven't yet been upgraded.

So, would you remove your wrongly applied earthing after RCDs have been fitted?

But I believe the regs disagree with you.

Please offer an example.

Bonding of conductive parts in a bathroom IS required unless certain conditions are met.

No. Bonding of extraneous-conductive-parts is required.
Overlooked the extraneous again.
That does not include isolated conductive parts.

That sounds very very much to me like those who wrote the regs consider that the electrician MUST cater for the potential of a live conductor randomly attacking a pipe.

Then you are mistaken.

 

[JohnW2]

So, taking the example I gave - and leave out any bonding/earthing of the pipework. Which circuit is not designed properly ? The RFC is designed properly - if there's a L-E fault in the circuit then the breaker will trip. The boiler supply is designed properly - if there's a L-E fault in it or the boiler then the breaker will trip and/or the fuse in the FCU/plug will blow. Connect the live in the larger circuit to the CPC in the other and it's not guaranteed.

Well, yes, but you are postulating an incredibly unlikely scenario - and not one that we are normally expected to cater for. Exactly how are you envisaging that a fault would develop between the L of a cooker circuit and CH pipework?

Without any pipework in the equation at all, you might just as well postulate a fault (again, goodness knows how) between the L of a 45A/50A shower circuit and the CPC of a 6A lighting circuit.

But I believe the regs disagree with you. Bonding of conductive parts in a bathroom IS required unless certain conditions are met. That sounds very very much to me like those who wrote the regs consider that the electrician MUST cater for the potential of a live conductor randomly attacking a pipe.

For a start, it is essentially only a 'legacy' consideration. For new installations, regs require everything in a bathroom to be RCD protected (and also that required MEB is in place in the premises), in which case (if pipework is metal), supplementary bonding will not be required. However, I think you are again confusing earthing and bonding. When supplementary bonding is required in bathrooms, it is to minimise the pd which can exist between pipework and exposed-conductive-parts of electrical equipment which have become 'live' due to a fault - not because of some hypothetical 'becoming live of the pipes'.

Kind Regards, John

 

[ban-all-sheds]

Because someone didn't cater for it, we now have live pipework with only the boiler wiring to "earth" it

And until/unless that happens you've connected things which are not part of the electrical installation to your wiring.

Bonding of conductive parts in a bathroom IS required unless certain conditions are met.

No.

Bonding of EXTRANEOUS conductive parts.

That sounds very very much to me like those who wrote the regs consider that the electrician MUST cater for the potential of a live conductor randomly attacking a pipe.

That would be earthing, not bonding.

 

[Cabbyfan]

The way I interpret it is, the regs state "main protecting bonding are required to connect extraneous-conductive parts to the main earthing terminal".

This includes "metallic installation pipes" plus gas pipes, heating oil pipes etc etc. It also refers to "internal pipework which may have been buried"

My onsite guide does state there is no requirement to bond incoming services where the pipe is plastic, blue for potable water for example. It then goes on to say "main bonding is recommended unless it has been confirmed that any metallic pipework within the building is not introducing earth potential"

To me having a load of copper pipework running in reasonable proximity to cables means it should be a main bond.

 

[JohnW2]

The way I interpret it is, the regs state "main protecting bonding are required to connect extraneous-conductive parts to the main earthing terminal".

That is true.

This includes "metallic installation pipes" plus gas pipes, heating oil pipes etc etc. It also refers to "internal pipework which may have been buried"

That is also true, although internal pipework buried (in the ground) is very rare.

My onsite guide does state there is no requirement to bond incoming services where the pipe is plastic, blue for potable water for example. It then goes on to say "main bonding is recommended unless it has been confirmed that any metallic pipework within the building is not introducing earth potential".

Again, all true and sensible. As I said, provided the incoming supply is plastic, it would be extremely unusual that any of the internal pipework could be 'introducing earth potential' (i.e. buried in soil somewhere in their path).

To me having a load of copper pipework running in reasonable proximity to cables means it should be a main bond.

That makes no sense. You have correctly indicated what needs main bonding, and that does not include 'pipes running in proximity to cables' If you did decide you wanted to connect them to earth, that would be earthing, not any sort of bonding (and certainly not main bonding). However, as I've said, in practice any metal pipework will nearly always already be earthed - by virtue of immersion heaters, boilers and central heating components etc.

Kind Regards, John

 

[ban-all-sheds]

To me having a load of copper pipework running in reasonable proximity to cables means it should be a main bond.

How does the pipework running in reasonable proximity to cables mean that it is introducing earth potential?

 

[EFLImpudence]

The way I interpret it is, the regs state "main protecting bonding are required to connect extraneous-conductive parts to the main earthing terminal".

Yes, as you have now included the word 'extraneous'.
So, NOT an e-c-p, no need to bond.

This includes "metallic installation pipes" plus gas pipes, heating oil pipes etc etc. It also refers to "internal pipework which may have been buried"

Yes, extraneous-conductive-parts.

My onsite guide does state there is no requirement to bond incoming services where the pipe is plastic, blue for potable water for example. It then goes on to say "main bonding is recommended unless it has been confirmed that any metallic pipework within the building is not introducing earth potential"

Yes, if NOT confirmed that it is NOT an extraneous-c-p.
A typical OSG one size fits all to be on the safe side.
To confirm that it was not an e-c-p you would have to disconnect from other parts and electrical connections which, in an existing installation, is not going to happen.
As has been said it will likely be earthed somewhere in the premises so bonding will not be detrimental.

To me having a load of copper pipework running in reasonable proximity to cables means it should be a main bond.

Now you are talking about EARTHING - but you are wrong.

 

[SimonH2]

Well, yes, but you are postulating an incredibly unlikely scenario - and not one that we are normally expected to cater for. Exactly how are you envisaging that a fault would develop between the L of a cooker circuit and CH pipework?

I stated one example - whoever is responsible, the cable ends up on/next to a hot pipe. It seems that many plumbers have "little respect" for cables. It doesn't have to be behind the cooker - it could well be under the floor between CU and kitchen.

Without any pipework in the equation at all, you might just as well postulate a fault (again, goodness knows how) between the L of a 45A/50A shower circuit and the CPC of a 6A lighting circuit.

That's less likely - though not impossible. I suppose someone could put a nail though which managed to touch the L in one cable and the CPC in the other - but that would be highly unlikely. Just having two cables running together would be very very unlikely to do it without some form of external influence causing damage.

But hot pipes and cables - I know I've seen it often enough. Are you telling me you haven't ?

 

[JohnW2]

Well, yes, but you are postulating an incredibly unlikely scenario - and not one that we are normally expected to cater for. Exactly how are you envisaging that a fault would develop between the L of a cooker circuit and CH pipework?

I stated one example - whoever is responsible, the cable ends up on/next to a hot pipe. It seems that many plumbers have "little respect" for cables. It doesn't have to be behind the cooker - it could well be under the floor between CU and kitchen.

I agree that anything is possible, but there is a limit to how far one can/should reasonably go in anticipating the incredibly improbable. In any event, yet again, if you did want to connect the pipework to earth, that would be earthing (to ensure operation of a protective device), not bonding (to equalise the pd between two conductors).

Without any pipework in the equation at all, you might just as well postulate a fault (again, goodness knows how) between the L of a 45A/50A shower circuit and the CPC of a 6A lighting circuit.

That's less likely - though not impossible. I suppose someone could put a nail though which managed to touch the L in one cable and the CPC in the other - but that would be highly unlikely. Just having two cables running together would be very very unlikely to do it without some form of external influence causing damage.

Two cables plus a hot pipe in close proximity? As I've said, these scenarios are all so incredibly improbable that only the ultra-cautious would even think of making provision for their occurrence.

But hot pipes and cables - I know I've seen it often enough. Are you telling me you haven't ?

Of course. In my house, I actually inherited some cables which were 'wound around' CH pipes that had clearly been like that for years, without anything catastrophic having happened. Do I take it that you have come across cases in which proximity of a cable to a hot pipe has resulted in its live conductor coming into contact with the pipe? I certainly haven't. ... and, again, if you decided you wanted to earth the pipe (which almost certainly would already be earthed), that would be earthing, not bonding.

Kind Regards, John

 

[ban-all-sheds]

Of course. In my house, I actually inherited some cables which were 'wound around' CH pipes that had clearly been like that for years, without anything catastrophic having happened.

Indeed.

PVC cables are designed to be perfectly fine with the conductors at 70°C.

Simon - what's the flow temperature for your CH system? More than 70°C?

 

[bernardgreen]

PVC cables are designed to be perfectly fine with the conductors at 70°C. Simon - what's the flow temperature for your CH system? More than 70°C?

If the conductor is dissipating heat due to the current flowing then the conductor will be at a higher temperature than the outside of the cable.

I don't know the thermal resistance of PVC so cannot guestimate how hot a conductor would be if the ambient air ( or a copper pipe ) was heating the outside of the cable to 70°C

 

[JohnW2]

PVC cables are designed to be perfectly fine with the conductors at 70°C. Simon - what's the flow temperature for your CH system? More than 70°C?

If the conductor is dissipating heat due to the current flowing then the conductor will be at a higher temperature than the outside of the cable. ... I don't know the thermal resistance of PVC so cannot guestimate how hot a conductor would be if the ambient air ( or a copper pipe ) was heating the outside of the cable to 70°C

It's not the conductor temperature which was the issue. Simon is seemingly concerned that heat from the pipe would be adequate enough to melt to sheath and insulation of the cable, after which the exposed live conductor might somehow come into contact with the pipe.

If PVC cables are deemed to be safe when operating continuously with a conductor temperature of 70°C, I think you can rest assured that the temperature of the PVC would have to be an awful lot higher than that for it to melt (IIRC, the melting point of PVC is around 160°C).

I think we may have found someone who is even more cautious and risk-averse than you

Kind Regards, John

 

[ban-all-sheds]

If the conductor is dissipating heat due to the current flowing then the conductor will be at a higher temperature than the outside of the cable.

I don't know the thermal resistance of PVC so cannot guestimate how hot a conductor would be if the ambient air ( or a copper pipe ) was heating the outside of the cable to 70°C

None of that is relevant.

The point is that if a conductor at 70°C will not melt a cable from the inside, a HW pipe at 70°C will not melt it from the outside.

And CH pipes aren't that hot anyway.

 

[JohnW2]

None of that is relevant. The point is that if a conductor at 70°C will not melt a cable from the inside, a HW pipe at 70°C will not melt it from the outside. And CH pipes aren't that hot anyway.

Quite so - you appear to have echoed my point.

Kind Regards, John

 

[JohnW2]

Taking a practical example. Gas and water both enter in plastic, so the argument goes that they don't need bonding. The only earth for the water and heating systems is via the boiler mains connection. A fault develops between the live of (say) the 32A circuit feeding the cooker or an RFC and a water or heating pipe. The only route to earth is therefore via the boiler mains supply terminal block, a length of possibly only 0.75mm^2 flex, and then whatever circuit the boiler feeds off back to the CU. The impedance of this circuit could quite feasibly be sufficient to allow a "significant' fault current to flow, raising the water & heating pipework to a dangerous potential, and also creating a risk of fire as the thin flex burns out

I forgot to add that, even in the incredibly improbable scenario you hypothesise, if you do the appropriate adiabatic calculation, you will find that, for typical situations (e.g. Zs=0.5-1.0Ω, PEFC=230A-460A), a 0.75mm² (or even 0.5mm²) CPC is usually going to be more than adequate for a circuit protected by a B32.

Kind Regards, John